#include <stdio.h>

main()
{
  int i;
  int n = 1000000000;
  double s = 0.0;
  system("date");
#pragma omp parallel for reduction(+:s) 
  for (i = 0;i < n;i++)
    s += (4.0 / (4 * i + 1) - 4.0 / (4 * i + 3));
  printf("%lf\n",s);
  system("date");
}